I was looking at ultrasonic flow meters, and it seems likely that the sensor itself can be made with just a couple of piezos, that measure time of flight in one direction and then the other and gauge flow rate that way, it may need it's own cpu (at least for prototyping) to send and process the signals, but it might be the path forward for good range, accuracy, and reliability, and very low cost, ease of manufacture, and compatibility with lots of fluids. Don't know if it has been considered yet. My bike is the only non-efi left so I'm not real gung ho about it, just thinking about it.
Ultrasonic flow meter - Wikipedia, the free encyclopedia